-LRB- Health.com -RRB- -- An essential nutrient found in fish oil does not appear to slow the mental decline associated with Alzheimer 's disease , according to a new study in the Journal of the American Medical Association .

The study is merely the latest to cast doubt on the mental benefits of the omega-3 fatty acid docosahexaenoic acid -LRB- DHA -RRB- , which until recently was considered a promising way to minimize the risk and damage of dementia . -LRB- The other main ingredient found in fish oil , eicosapentaenoic acid , or EPA , is not believed to play a significant role in brain health . -RRB-

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DHA or fish-oil supplements are n't likely to cause any harm to Alzheimer 's patients , but they are n't likely to do any good either , says Steven H. Ferris , Ph.D. , the director of the Aging and Dementia Research Center at New York University .

Fish oil `` seems to be healthy in general , and maybe for other things it 's helpful , but it does n't benefit cognitive function in a person with Alzheimer 's , '' says Ferris , who was not involved in the study .

Research on DHA has been inconclusive and sometimes conflicting . Several studies that followed large groups of people as they aged have suggested that a diet rich in fish is linked to a reduced risk of dementia and mental decline , but most randomized controlled trials comparing DHA supplements with placebo have found no benefit .

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`` It 's not the first time something in large epidemiological datasets just did n't work out clinically , '' Ferris says , noting that statin medications , anti-inflammatory drugs , and estrogen therapy have all failed to live up to their initial promise in preventing or treating Alzheimer 's . `` Unfortunately , that seems to be the situation here . ''

The study was funded by the National Institute on Aging and was led by Dr. Joseph F. Quinn , M.D. , a neurologist at the Oregon Health and Science University , in Portland .

Quinn and his colleagues randomly assigned about 400 women and men in their mid-70s with likely Alzheimer 's disease -- the disease is very difficult to accurately diagnose -- to take 2 grams of DHA or placebo capsules per day . After 18 months , the average mental decline in the DHA and placebo groups was nearly identical , as measured on two separate tests and rating scales .

Despite the disappointing results , the study does n't entirely rule out the possibility that DHA may have some benefit if taken earlier in life . A growing body of research suggests that dementia begins decades before any noticeable symptoms surface , and it 's possible that DHA helps prevent or slow those harmful changes .

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Treatments such as DHA may be too little too late for people who are already showing signs of Alzheimer 's , according to Dr. Kristine Yaffe , M.D. , a professor of psychiatry at the University of California , San Francisco .

`` Effective treatment strategies to prevent progression of -LSB- Alzheimer 's disease -RSB- will likely need to be initiated earlier , '' Yaffe writes in an editorial accompanying the study .

Copyright Health Magazine 2011

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Studies show that an essential nutrient in fish oil does not appear to slow mental decline

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Fish oil seems to be healthy in general , but does not benefit Alzheimer 's patients cognitively

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Treatments such as DHA could prevent or slow dementia if started earlier in life